There have been various processes of assembling a vehicle body of an automobile, in which the vehicle body is carried while being mounted on an overhead conveyor, and underbody parts such as an engine mission assembly or a suspension assembly are loaded to the vehicle body. Among such processes, there is a process that uses a slat conveyor, which is placed on the factory floor so that the slat conveyor is aligned parallel to the direction of the work flow, transports a carriage carrying the underbody parts along with a vehicle body that is transported by an overhead conveyor, and elevates this carriage and the underbody parts at a predetermined location to load the underbody parts to the vehicle body (for example, refer to Patent Document 1). The slat conveyor is supported by a base frame placed in an underground pit beneath the factory floor. A carriage returning device is placed within the underground pit. This carriage returning device lowers the carriage, which has reached a downstream side of the slat conveyor, to a location below the factory floor. Further, this carriage returning device transports the carriage to an upstream side via an underground space below the slat conveyor, and elevates the carriage again above the factory floor.
In addition, Patent Document 1 discloses a synchronized transportation device that changes the position of a carriage with respect to a slat conveyor according to the type of vehicle (wheel base). This change in the positioning of the carriage is made based on a vehicle body that is placed on an overhead conveyor which is synchronized with the slat conveyor. Further, the synchronized transportation device according to Patent Document 1 aims to change the position at which underbody parts are loaded to the vehicle body based on the type of the vehicle.
More specifically, the above synchronized transportation device includes a frame and a dog affixed to the slat conveyor, multiple start signaling switches which operate based on the dog and correspond to the vehicle types, a clamp unit which is placed on the carriage and clamps certain places of the frame, a carriage stopper device which can stop the transportation of the carriage by the slat conveyor, and an upper level computer that transmits data of vehicle types. Each of the start signaling switches can detect the dog. However, based on the data of vehicle types transmitted from the upper level computer, only the start signaling switch which corresponds to a particular vehicle type will detect the dog. At this time, the clamp unit placed on both sides of the carriage clamps the frame placed on both sides of the slat conveyor. Further, when the start signaling switch which corresponds to the particular vehicle type detects the dog while the carriage is being stopped by the carriage stopper device, each clamp unit on the carriage clamps each frame on the slat conveyor. At the same time, the carriage, which had been halted by the carriage stopper device, starts moving. Thus, the carriage and the underbody parts are transported by the slat conveyor while being positioned according to vehicle type at a certain position with respect to the vehicle body placed on the overhead conveyor.